1. Field of the Invention
This invention relates to a motor controller for performing drive control based on the PWM (pulse width modulation) system, for the purpose of protecting amplifiers and motors against anomalous currents and achieving high operating efficiency therein.
2. Description of the Related Art
PWM is a known system for controlling AC motors such as induced-current three-phase synchronous motors and permanent magnet synchronous motors. When control is performed with the PWM system, the voltage applied to the motor is actually varied by altering the on-off time ratio of such switching elements as a drive transistors in a power conversion circuit.
When controlling a motor using such the PWM system, there are cases where the actual current being supplied to the motor becomes abnormally high for an instant relative to a current command. There are a number of primary causes of these anomalous currents, including (a) the current exhibiting intermittent oscillation due to the current control gain being set high, resulting in spikes in the actual current, (b) noise being added to the feedback current fed back to the control circuit from the motor, resulting in spikes in the actual current, and (c) inadequate voltage for controlling motor current when the motor is turning at high speed, making it impossible to obtain the designed control characteristics in the current control loop, resulting in control instability and spikes in the actual current.
When the actual current increases abnormally as described above, anomalous currents flow to the amplifier and/or motor, creating a danger of failures in the power devices configuring the amplifier, the amplifier or motor wiring, or, in the case of permanent magnet synchronous motors, motor magnet demagnetization.
For these reasons, protective circuits have been provided in conventional motor control apparatuses to cope with such anomalous currents. These protective circuits compare the actual current with a fixed threshold value which was set for an anomalous currents and, when the actual current exceeds the threshold value, interrupt, in all phases, the PWM commands being sent to the power conversion circuit. Thus all current supply to the motor is cut off and the apparatus is put into an alarm status.
With such a protective circuit as this, it is possible to protect the amplifier power devices and the motor magnets, but, the current supplied to the motor is cut off. For this reason, the overall motor control apparatus is placed in an alarm generation status, and the apparatus being driven by the motor ceases to be under control.
A motor is ordinarily provided with a dynamic brake that runs current from the motor through a resistance and performs a stopping action. Therefore, when the protective circuit is activated and the supply of power to the motor is interrupted, an alarm status is entered wherein the entire system inclusive of the motor is stopped.
Usually, in an alarm status, not only is control of a motor stopped, but also entire system is stopped and kept uncontrolled. Accordingly, once this alarm status is in effect, it becomes impossible either to continue processing or movement, or to perform control. After this alarm status has been entered, moreover, it becomes necessary to restart the system and/or program, which requires operator intervention and results in a decline in the overall system operating rate.
Also, when the braking force of the dynamic brake is smaller than the torque required for deceleration, this can be hazardous, resulting even in work or stroke-end collisions.